Abstract

In order to investigate the behavior of slab-derived volatiles in the subduction environment,
helium isotope ratios have been measured in geothermal gases from the Tabar-Lihir-TangaFeni
(TLTF) chain in the Bismarck Archipelago of Papua New Guinea. As recorded by several
geochemical tracers, these volcanos carry an exceptionally large slab-derived component, and therefore
may provide new insights to the old question of volatiles in subduction zones. Geothermal
gases from Lihir Island have homogeneous ^3He/^4He ratios of 7.18±0.07 times the atmospheric ratio
(R_A), while those from Ambitle Island (Feni Group) have lower ratios of 6.61±0.13 R_A. These
^3He/^4He ratios are within the range defined by more-typical arc volcanos, but lie at the low end of
the spectrum observed in arc volcanos erupted through purely oceanic crust. Although a small slab-derived
signature (^3He/^4He ratio lower than depleted mantle) exists in the TLTF gases, these data
demonstrate that even in volcanos with a comparatively large slab component, He is overwhelmingly
derived from the depleted mantle wedge. This observation further confirms the relative insensitivity
of He isotopes to the presence of slab fluids.
He isotope ratios of 6.25 R_A were measured in geothermal gases from the Rabaul Caldera
on New Britain Island. Coincidentally, these samples were taken six months prior to the major
1994 eruption at Rabaul. In conjunction with samples taken from the same locality 8 years earlier,
these data allow us to test whether increasing He isotope ratios associated with fresh ascending
magmas precede volcanic eruptions. Although some of the 1986 samples had much lower ^3He/^4He
ratios (5 R_A than observed in 1994, one did not. We thus find no strong evidence for a systematic
rise in the He isotope ratio of the Rabaul fluids between 1986 and 1994. If a ^3He/^4He increase did
precede the Rabaul eruption, then it occurred either prior to 1986 or sometime between our 1994
sampling and the eruption.